Black Hill is a famous locality for agates of the “sagenitic Typus” - in this case the nodules are containing pseudomorphs of chalcedony after fine fibrous crystallized zeolithic minerals.
More informations about scottish agates are given in Dr.H.G. Macpherson´s book “agates” (published by the British Museum (Natural History) and the National Museums of Scotland).
You can find more informations about scottish agates on the following webpages:
Chris Harlows webpage about scottish agates
Nick Crawfords webpage scotsagate
H.Currie Minerals of Scotland